What are scientific revolutions and how are they accomplished? Jürgen Renn's talk at the DESY Physics Colloquium focuses on the long history of general relativity in the twentieth century as a case study and argues that scientific revolutions are not sudden paradigm shifts but protracted processes that may stretch over several generations. Older theories are not simply discarded but serve, in the course of their elaboration, as scaffolding for the emergence of a new system of knowledge. The talk emphasizes the role of borderline problems between different conceptual systems as triggers driving structural changes of systems of knowledge.